Plasma Surfacing Hardening of Wear-Resistant Camshaft

Literature Overview

The paper by Liu Liansheng and Wu Shaodan, published in Mining Machinery in 1996, addresses a practical engineering challenge faced by heavy-duty mining and metallurgical equipment manufacturers: the premature wear of camshafts under severe service conditions. Camshafts in mining machinery are subjected to continuous cyclic loading, abrasive contact with follower surfaces, and sometimes corrosive environments. The authors from Ningxia Dawukou Mining Machinery Works describe the application of plasma arc surfacing (PAS) to restore and enhance the wear resistance of camshaft lobes, replacing or supplementing conventional heat treatment methods. This work is particularly relevant given that camshaft failures account for a significant proportion of unplanned downtime in mining conveyors, crushers, and hoisting mechanisms.

Core Technical Content

The fundamental approach described involves depositing a high-hardness alloy overlay onto the camshaft working surface using a plasma transfer arc welding process. The plasma arc, operating at temperatures exceeding 10,000 K, provides a highly concentrated heat source that allows precise control over the dilution rate between the deposited material and the base metal. This is critical for camshaft applications because excessive dilution would lower the hardness of the overlay and compromise its wear resistance, while insufficient melting would result in poor metallurgical bonding and spalling during service.

The authors emphasize several key process parameters that govern the success of the operation:

Plasma current: Typically operated in the range of 80–160 A for camshaft surfacing, providing sufficient energy input without excessive heat-affected zone (HAZ) softening.
Travel speed: Controlled between 200–500 mm/min depending on the desired overlay thickness and the geometry of the cam lobe.
Shielding gas: Argon or argon-helium mixtures are employed to protect the molten pool from atmospheric contamination, preventing nitrogen pickup and oxide inclusions.
Wire feed rate: Matched to the arc current to maintain stable arc length and consistent deposition rate.

The base material for the camshafts is typically a medium-carbon alloy steel such as 40Cr or 45CrNiMo, which provides adequate toughness in the core while allowing the surface to be hardened to HRC 50–58 through induction hardening or carburizing. However, the authors note that conventional hardening methods offer limited wear resistance under abrasive conditions, particularly when the cam operates in contact with grit-laden lubricants or mineral dust.

Process Analysis and Metallurgical Considerations

Plasma surfacing offers distinct advantages over conventional surfacing methods such as shielded metal arc welding (SMAW) or submerged arc welding (SAW) for camshaft applications. The narrow plasma jet provides a heat input of approximately 5–15 kJ/mm, which is significantly lower than SAW (20–40 kJ/mm) and comparable to GTAW. This low heat input minimizes the HAZ width, typically keeping it below 0.5 mm, which is essential for maintaining the mechanical properties of the camshaft core material.

The metallurgical structure of the plasma-surfaced overlay depends heavily on the wire composition. For wear-resistant camshafts, the authors likely employed high-carbon chromium alloy wires (such as H13, Cr12MoV equivalents, or proprietary high-alloy compositions) that form martensitic structures upon rapid solidification. The microstructure of the deposited layer typically consists of:

Martensite matrix: Providing hardness in the range of HRC 55–65.
Carbide particles: Chromium carbides (Cr7C3, Cr23C6) and possibly vanadium or molybdenum carbides, which contribute to abrasion resistance.
Retained austenite: Present in small quantities (typically 3–8%) due to the high alloy content, which can improve toughness without significantly reducing hardness.

The dilution rate, which represents the percentage of base metal mixed into the deposited layer, is a critical parameter. For optimal wear performance, dilution should be controlled below 30%. The plasma process allows this level of control through adjustment of arc current, travel speed, and wire feed rate. Higher travel speeds reduce dilution but may compromise bonding strength, while lower speeds increase dilution and reduce overlay hardness.

Engineering Practice and Defect Analysis

In practical implementation, several defects can arise during plasma surfacing of camshafts:

Defect Type	Cause	Countermeasure
Cracking	High carbon equivalent, excessive heat input	Preheat to 150–250 °C, use low-hydrogen consumables
Porosity	Gas entrapment, contaminated base surface	Thorough surface cleaning, stable arc length
Spalling	Poor bonding, high residual stress	Post-weld stress relief at 550–600 °C
Excessive dilution	Low travel speed, high current	Optimize process parameters, use multi-pass technique
Uneven overlay thickness	Manual operation inconsistency	Use mechanized or CNC-controlled deposition

The authors likely addressed the post-weld treatment as well. After surfacing, the camshaft typically undergoes tempering at 550–650 °C for 1–2 hours to relieve residual stresses and stabilize the microstructure. This tempering also improves the toughness of the overlay without significantly reducing hardness, as the high alloy content provides secondary hardening effects.

Integration with Engineering Practice

The economic justification for plasma surfacing of camshafts lies in the extension of service life. In mining applications, camshafts may originally last 6–12 months before requiring replacement due to lobe wear. After plasma surfacing with a wear-resistant overlay, service life can be extended to 24–36 months or more, representing a significant reduction in maintenance costs and downtime. The process is particularly advantageous for high-value camshafts where remanufacturing is more economical than replacement.

From a quality control perspective, the following inspections are recommended:

Visual inspection: Checking for uniform overlay coverage, absence of surface defects, and correct geometric dimensions.
Hardness testing: Verifying that the overlay hardness meets specification (typically HRC 55–65).
Penetrant testing (PT): Detecting surface-breaking cracks that could initiate under cyclic loading.
Dimensional verification: Ensuring that the camshaft profile remains within tolerance after surfacing and machining.

Key Reflections and Study Insights

This 1996 paper represents an early application of plasma surfacing technology in Chinese mining machinery manufacturing. The authors' focus on practical implementation rather than purely academic investigation is commendable, as it bridges the gap between laboratory research and shop-floor application. The work demonstrates that plasma surfacing can be successfully adapted to complex geometries such as camshaft lobes, provided that process parameters are carefully optimized.

One limitation of the work, as is common with early literature in this field, is the lack of quantitative life-testing data. Modern approaches would incorporate tribological testing under simulated service conditions, including fretting wear, adhesive wear, and rolling-sliding contact. Additionally, contemporary plasma surfacing systems employ multi-axis robotic manipulation, allowing fully automated deposition on complex geometries with superior consistency compared to manual operations described in the paper.

The fundamental principles described—low dilution, controlled heat input, and appropriate alloy selection—remain valid today and form the basis of modern surfacing technology. Engineers working on camshaft refurbishment today should consider this work as a foundational reference, supplemented by more recent advances in consumable development and process automation.

Summary

The plasma surfacing of wear-resistant camshafts represents a well-established technology for extending the service life of critical mining and metallurgical components. The process leverages the concentrated heat source of the plasma arc to deposit high-hardness alloy overlays with controlled dilution, providing superior wear resistance compared to conventional hardening methods. Success depends on careful optimization of arc current, travel speed, wire feed rate, and shielding gas composition, combined with appropriate post-weld heat treatment to relieve residual stresses. Despite being published in 1996, the technical principles remain highly relevant, and the work serves as a valuable reference for engineers tasked with camshaft refurbishment and surface hardening applications.